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In Reply to: PPSL posted by djk on May 16, 2003 at 17:26:18:
Hi Dennis!I agree with others on this thread - I'd like to see more discussion of this. Can you show some more detail and maybe link some references?
I can see how second harmonics would be suppressed if they were made to be out of phase. And so I can see some merit in the idea - Sort of like complementary-symmetry amps. But the thing I'm wondering about is how do you get the anomalous harmonics to be 180o out of phase while having the fundamental in phase. In an amplifier, this is done in the coupling mechanism, where the two out-of-phase signals are combined to be in-phase. It creates a condition for common-mode rejection, where the artifacts cancel out. But in a loudspeaker, no similar mechanism can be employed that doesn't cancel the fundamental. At least, that's where I'm drawing a blank on this one.
Seems to me that if two similar motors are used, the phase relationship between these components would be the same. Even if you drive the two diaphragms in opposite directions, if you house them in a cabinet that combines their pressure waves for summation then the results are the same. And so then when you connect them so that the pressure wave of each creates summing pressure - as opposed to cancelling pressure - then it would seem that the same relationship would be maintained for any anomalous harmonics that were generated.
I mean, I think that if there are non-linearities that are made by the mechanical suspension - something that, in effect, causes a sort of rectification-like property whereby the cone actually goes further in one direction than it does in the other - I could see how this push-pull arrangement might reduce second harmonics. This sort of non-linearity is, after all, what causes second-order artifacts, so you might be able to cancel the effects of high-excursion mechanical suspension non-linearities with this configuration. But the thing I'm not understanding is that I would expect that sort of distortion to be onset at high levels, and not down low.
Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders, and that's a different issue entirely, different causes and so requiring different solutions. But then again, the voice coil energy is polarized the same as the magnet on one half cycle, and opposite on the other, so I suppose that's a condition that tends to increase second-orders as well. I guess I need to do some reading about these kinds of systems, because I'm completely at a loss to this sort of arrangement. It's all speculation on my part.
So I'd appreciate any links you could share that I could peruse. I agree with you 100% on the big basshorn thing. It would be great to be able to have 'em, but horns for low frequencies must necessarily be large. So for deep bass from speakers that aren't as big as a house requires other technologies. Horns are great down to 40Hz, but below that they become pretty big. You just can't make a basshorn have 30Hz f3 in 20 cubic feet or less.
Wayne
Follow Ups:
"Can you show some more detail and maybe link some references?"At the time I was looking at both the EV MTL4 and the KEF 104, both had their strengths and their weaknesses. I tried to get the best of both worlds.
EV no longer has the datsheets for the MTL4 on line, and the KEF 104 has been discontinued for awhile too.
"But the thing I'm wondering about is how do you get the anomalous harmonics to be 180o out of phase while having the fundamental in phase."
The wavelength of 200hz is ~68", my first model had the magnet structure of the inverted driver only 1" away from the front of the other cone. The cones are so close together and the wavelengths so long that it still subtracts.
"Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders"
The 3rd harmonic reduction is a function of the low-pass behavior of the plenum. IE: if the plenum acts like a 12dB low pass at 200hz it will roll off the 3rd harmonic of 100hz by roughly 6dB. Obviously no useable reduction can occur much below 80hz or so.
Two interesting stories:
On a box that had the woofers hooked up wrong, ie: in phase electrically, the woofers would just flap and absolutely NO sound could be heard.
Car stereo installer just finishes carpeting a 4 X 12 box, wants to check it out before the customer shows up. He's running a 30W Pyramid car stereo amp full range, no 6th order EQ, no HF, no crossover. It sounds real wimpy in his 30 X 24 garage, he's worried. I hooked up the electronic crossover I brought over that I added 6th order EQ to, along with his 100W Fisher/Sanyo garage system, and the same 30W Pyramid amp. He's stunned, it really thumps now, and can play loud enough to rot your face off. Goes into a Ford Escort with 1.2KW on it, played so loud standing in the garage that I refused to get in the car.
Hello again Dennis!This configuration looks interesting, and I'm starting to look at it closely. The π cornerhorn can easily be built with dual woofers and it has a plenium of sorts. Since the woofers aren't exposed, it would not be as unattractive as a baffle-mounted setup. Then again, baffle-mounted speakers can be built that have one woofer only run to 100Hz or so, to reduce interference anomalies in the lower midrange. Those are just a couple of my thoughts for implementations of this configuration in addition to the plenium you've described.
You wrote:
> > EV no longer has the datsheets for the MTL4 on line, and the
> > KEF 104 has been discontinued for awhile too.I assume these two manufacturers used this configuration, yes? I wonder why these models were discontinued. It might have been aesthetics - Speakers made this way might be perceived as being "peculiar looking."
A quick search on the internet shows that EAW is using the push/pull woofer configuration , and I also notice that they use some other design techniques that I personally find attractive.
I appreciate your discussions here and I'll check into this some more.
I wrote:
"But the thing I'm wondering about is how do you get the anomalous harmonics to be 180 o out of phase while having the fundamental in phase."
And you replied:
> > The wavelength of 200hz is ~68", my first model had the magnet
> > structure of the inverted driver only 1" away from the front of
> > the other cone. The cones are so close together and the
> > wavelengths so long that it still subtracts.I wasn't worried about distance problems shifting the phase because the frequencies of interest are second harmonics of the woofer's resonant frequency region. You've rightly shown just how long these wavelengths are.
More what I was concerned with was a confirmation that the second harmonics would be generated exactly in phase with the fundamental as opposed to 180 o out of phase. I wouldn't expect any other phase relationship, but it would not be unreasonable to expect either or both variants, and I'm not sure at what proportions.
For example, if the suspension were more resistive in the forward direction than the reverse on one cone and just the opposite on the other, then I would expect second harmonics to be of opposite phase in these two speakers. In that case, it would actually perform better to have the woofers connected in the "normal" configuration, as this would cancel the harmonics due to their opposite phase relationship.
Then again, this would be an example of poor quality control and I'm not sure that's the sort of issue we're dealing with here. I suspect it has more to do with non-linearities near excursion limits, and where the amplitude of the voice coil's magnetic polarity becomes significant.
I wrote:
"Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders."
And you replied:
> > The 3rd harmonic reduction is a function of the low-pass behavior
> > of the plenum. IE: if the plenum acts like a 12dB low pass at
> > 200hz it will roll off the 3rd harmonic of 100hz by roughly 6dB.
> > Obviously no useable reduction can occur much below 80hz or so.Yes, the configuration can do nothing about odd-order harmonics. This is very similar to a complementary-symmetry amplifier configuration, which have benefits in the reduction of even-orders. I understand what you're saying about the acoustic chamber, but in regards to the summing of the two speakers, we're talking about even-order improvements here.
Wayne
EAW makes a lot of nice stuff.At one time they were the sole OEM source for RCF in the USA. At one time I was on a scheduled release program and getting 250pc price breaks. N280 and N480 compression drivers. The N480 diaphragm fits 1" JBL structures(2410/2420/etc), sounds better, costs less, and is more rugged.
"I wonder why these models were discontinued."
KEF was sold in 1992, floundering even before the death of Raymond Cooke(1996), I can't remember what year Laurie Fincham left. Here is a description of their push-pull 4th order bandpass design:
The Dual Cavity Woofer System
Model 103/4 incorporates two 160mm woofers mounted in double coupled cavity configuration and linked with a force cancelling rod. This metal rod couples and thereby cancels the drive unit’s identical, but opposing, mechanical vibrations, whilst reducing distortion. Linking the woofers also prevents the transfer of energy to the main enclosure, a cause of the delayed resonances which often give rise the ‘boxy’ colouration thus reducing musical clarity. Model 103/4’s entire bass output is radiated by a smoothly contoured duct placed below the Uni-Q unit. This acts as an air diaphragm of very low mass, with an effective diameter similar to that of the midrange unit. Thus directional characteristics match, ensuring exceptionally smooth acoustic integration throughout the entire frequency range.http://www.kef.com/kefimages/image240.gif
(in case the image doesn't load)
The 103/104/105 used dual 6.5/8/10 woofers. All were a 50hz~150hz BP4 and with the KUBE had a Q=.5 20hz cut-off. The 107 had dual 10s, a bigger box, less effficent, and would do 20hz without the KUBE. The 105/107 had the MF/HF in pods atop the woofer cabinet, ala the B&W 801. The version of the 103 that only had one 6.5" mid sounded really good, but wouldn't handle enough power to get good 'n loud. The 104 almost got loud enough, but sounded funny in the MF. The 105/107 were too expensive(in 1988 a pair of 103s were about USD$1800
depending on finish, I think the 107s could get up to 10K$)A friend of mine, Bruce Knight, went to work with dB Sound in the early 80s. Bruce told me about these four 18" driver manifold boxes that had as much low end as their much bigger four 15" horn boxes.
• Manifold Technology •At the heart of these systems are the world renowned Electro-Voice“MT” enclosures. The “Manifold Technology” enclosures were co-developed by EV and db Sound to combine all of the sonic and cost advantages of reducing system size.
http://www.dbsound.com/images/MTStkLgo.jpg
Here's Bruce's small convex arrayed system for Aerosmith:
http://www.dbsound.com/images/Aeropic1.jpg
When Bruce did our hometown with Aerosmith he brought the 80,000 seat stadium rig into our 10,000 seat auditorium. Jackal opened up with their 'Chain Saw Boogie', I've run jackhammers that were quieter and had less impact that their mic'd chainsaw. I couldn't handle it, even with plugs.
The typical stadium system consists of 54, EV MTH4, Mid/Hi enclosures, 54, MTL4, Lo enclosures, and 16, MT2, Hi and Lo enclosures(a total of 248, 18" woofers). 56 Crest 8001 amplifiers are utilized to provide the system with 168,000 watts of power.
I told Bruce it was stupid to run a convex array, and that trap cabinets ought to face in, not out. He said it would be an all day afair to rig the fly-bars, but he would try it when he had an extra day to set up the gig.
"db Sound has taken the development of the MT system one step further with the introduction of the unique Concave Line Array. By arranging the enclosures in 40° opposing angles to one another all lobbing is eliminated horizontally. This results in 180° of side to side coverage without the “dead” or “hot spots” that are normally experienced with other systems.
The system effortlessly produced levels of 126db at the mix site as well as 110db, 450 feet from the stage, with no delay towers. Implementing a combination of proven acoustical principals, the Concave Line Array is virtually unrivaled anywhere in the world.
The system was critically acclaimed by the press as “the best outdoor sound system ever heard”."
http://www.dbsound.com/images/CLACntr.jpg
A a 1,200 seat club gig the MTL2 rig was being loaded out and I was waiting to load in (you can move the MTL2 without a forklift and get them up a fire escape without killing yourself). I suggested we grab some breakfast and let the stage hands do all the hard work, and afterwards he could hear what a 'real' system sounded like....TBC
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